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A Long-Lived Relativistic Electron Storage Ring Embedded in Earth’s Outer Van Allen Belt

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Science  12 Apr 2013:
Vol. 340, Issue 6129, pp. 186-190
DOI: 10.1126/science.1233518

Van Allen Variation

The two rings of relativistic particles called Van Allen Belts that encircle Earth were discovered during the space age, and are known to pose risks to satellites in geostationary orbit. NASA launched twin spacecraft, the Van Allen Probes, on 30 August 2012 to measure and characterize Earth's radiation belt regions. Baker et al. (p. 186, published online 28 February) have shown that a third, unexpected and temporary, radiation belt formed on 2 September 2012 to disappear 4 weeks later in response to changes in the solar wind.

Abstract

Since their discovery more than 50 years ago, Earth’s Van Allen radiation belts have been considered to consist of two distinct zones of trapped, highly energetic charged particles. The outer zone is composed predominantly of megaelectron volt (MeV) electrons that wax and wane in intensity on time scales ranging from hours to days, depending primarily on external forcing by the solar wind. The spatially separated inner zone is composed of commingled high-energy electrons and very energetic positive ions (mostly protons), the latter being stable in intensity levels over years to decades. In situ energy-specific and temporally resolved spacecraft observations reveal an isolated third ring, or torus, of high-energy (>2 MeV) electrons that formed on 2 September 2012 and persisted largely unchanged in the geocentric radial range of 3.0 to ~3.5 Earth radii for more than 4 weeks before being disrupted (and virtually annihilated) by a powerful interplanetary shock wave passage.

The magnetically confined radiation zones surrounding Earth were the first major discovery of the Space Age in 1958 (14). Long-term observations of these energetic particle populations have subsequently shown dramatic, highly dynamic changes of the outer Van Allen belt. Previous, rather sparse measurements of the radiation environment suggested that powerful acceleration events for relativistic electrons occur on time scales ranging from minutes (5, 6) to many hours (7, 8). Thus, there has been direct, as well as circumstantial evidence that an immensely powerful and efficient accelerator operates within the terrestrial magnetosphere just a few thousand kilometers above Earth’s surface.

On 30 August 2012, twin NASA spacecraft, the Radiation Belt Storm Probes (RBSPs), were launched into highly elliptical, low-inclination orbits around Earth. The RBSP satellites are fully instrumented with identical energetic particle, plasma, magnetic field, and plasma wave sensors to measure and thoroughly characterize the radiation belt regions (9). The scientific payloads on board the RBSP spacecraft (renamed the “Van Allen Probes mission” by NASA at a formal ceremony on 9 November 2012) have unprecedented detection sensitivity, energy resolution, and temporal sampling capability. In particular, the Relativistic Electron-Proton Telescope (REPT) experiment (10) measures the key ~1- to ~20-MeV electron population throughout the RBSP orbit, which extends from geocentric distances of radius r = 1.2RE to 5.8RE (RE: Earth radius = 6372 km). The REPT sensors were among the first instruments turned on and have been returning nearly continuous data from both Van Allen Probes spacecraft since 1 September 2012.

Prior key measurements of Earth’s radiation environment have been made (1113), but some of the longest and most comprehensive radiation belt observations have previously come from sensors on board the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) mission (14). This spacecraft made low-Earth orbit observations of inner- and outer-zone particles from its launch in July 1992 until its recent atmospheric reentry and demise on 13 November 2012 (15, 16). SAMPEX measured energy E > 1 MeV electrons at the near-Earth foot of magnetic field lines but was never able to look into the “throat” of the radiation belt accelerator in the magnetospheric equatorial plane. This contrasts dramatically with the REPT-A and REPT-B instrument data collected by the Van Allen Probes from 1 September 2012 through early October 2012 (Fig. 1). These data show that a powerful electron acceleration event was already in progress when the instruments were first turned on. The entire outer radiation belt was enhanced in electron flux from E ~ 3.0 MeV (Fig. 1A) up to energies well above the 5.0 ≤ E ≤ 6.2 MeV channel (Fig. 1C). At this time, the radiation belt populations clearly had the expected double-belt structure with an inner zone, an outer zone, and a “slot” region of greatly diminished intensity separating the two.

Fig. 1 Relativistic Electron Probe Telescope data.

Energetic electron data from the RBSP satellites in eccentric orbits around Earth showing several discrete energy channels of the REPT instruments on board the spatially separated RBSP-A and RBSP-B spacecraft. Each panel’s left y axis shows the L* parameter; the x axis shows time from 1 September to 4 October 2012. Electron differential flux values (in units of electrons per square centimeter·second·steradian·megaelectron volt) are in a color-coded logarithmic scale as shown to the right of the figure. (A) Electrons in the energy range 3.2 ≤ E ≤ 4.0 MeV. (B) Electrons with 4.0 ≤ E ≤ 5.0 MeV. (C) Electrons with 5.0 ≤ E ≤ 6.2 MeV.

What is most notable (and unexpected) is the clear emergence of a separate, previously unseen belt, or “storage ring,” of high-energy electrons that stands out clearly after 2 September 2012. This belt is evident in the E = 4.0 to 5.0 MeV range (Fig. 1B) and is the dominant flux feature in the E = 5.0 to 6.2 MeV energy range (Fig. 1C). This distinctive ring of highly relativistic electrons persists, changing only gradually, until its abrupt and almost complete disappearance late on 1 October 2012. Though the inner zone, the slot region, and the relativistic storage ring (3.0 < L* < ~3.5, where L* is the distance in Earth radii at which a magnetic field line crosses the magnetic equatorial plane) change relatively little over this 4-week period, the more distant part of the outer Van Allen belt shows huge dynamical changes with new electron populations appearing at L* > 4.0, beginning on ~7 September and intensifying greatly over a period of 2 weeks. Subsequently, the outermost parts of the outer Van Allen zone grew and diminished further with little effect on the storage-ring feature until the abrupt demise of virtually the entire outer-zone electron population at the end of 1 October. Other electron sensor systems on board the Van Allen Probes spacecraft, overlapping partially in energy coverage with the REPT sensors, also detected the storage-ring feature (17).

The distinct storage-ring feature is more clearly evident in the meridional plane projection of 4.0- to 5.0-MeV electrons from the combined REPT-A and REPT-B instrument records (Fig. 2). In the earliest observational phase (1 to 3 September 2012), the expected two-belt structure of the Van Allen zones is clear (Fig. 2A). In the next phase from 3 to 6 September, the relativistic storage ring was formed (Fig. 2B), probably largely by erosion and loss of the more distant parts of the outer zone. The storage ring persisted in a notably stable fashion (Fig. 2, C and D) throughout the remainder of September until its almost complete annihilation in early October 2012.

Fig. 2 Meridional plane projections.

Projections of the REPT-A and REPT-B electron flux (4.0 to 5.0 MeV) values, as shown according to the logarithmic color scale to the right of the figure. Each panel shows a limited interval of time in a magnetic latitude L* coordinate system. (A) For 1 to 3 September 2012, the expected two-belt Van Allen zone structure consists of an inner-zone electron population (L* < ~2.5), a relatively empty slot region (2.5 < L* < 3.0), and an outer-zone population (L* > 3.0). (B) From 3 to 6 September, only an intense belt of electrons remains in the range 3.0 < L* < 3.5; the inner zone and traditional slot region have not changed. (C) The storage-ring belt, or torus, feature persists at 3.0 < L* < 3.5, whereas a new slot region is seen at 3.5 < L* < 3.8, and a completely new outer-zone population has formed at L* > 3.8. (D) The storage-ring feature remains, whereas the outer zone at L* > 3.8 decays away. (E) The entire outer zone (L* > ~3.0) has virtually disappeared at these energies.

Instruments on board the Combined Release and Radiation Effects Satellite (CRRES) spacecraft (13) observed a powerful “injection” of high-energy electrons and protons deep into the inner part of Earth’s magnetosphere on 24 March 1991 (5, 18, 19). This was a highly impulsive event caused by an exceptionally strong interplanetary shock wave (5, 6). This event is a stark example of the sudden appearance of a newly energized population of both protons and electrons in a localized portion of the slot region of the radiation belts that is normally almost devoid of very energetic particles (19, 20). Moreover, this prior event contrasts with the storage-ring feature observed by the Van Allen Probes sensors: The storage ring clearly resulted largely from loss of the more distant portion of the outer-zone electron population rather than fresh, localized injection of the March 1991 type. The original acceleration of the electron population (before the turn-on of REPT on 1 September 2012) that eventually formed the storage ring may have resulted from local wave heating (21, 22), enhanced radial diffusion (23, 24), or both.

Based on prior radiation belt research [e.g., (7, 15)], the outer Van Allen zone electron populations would be expected to respond rather directly to changes in the solar wind, interplanetary magnetic field (IMF), and geomagnetic activity. The development of the storage-ring feature itself (Fig. 3) was closely associated with the loss of outer-belt electrons after passage of an interplanetary shock wave on 3 September 2012, seen as a sharp increase in solar wind speed (Fig. 3B) and abrupt change in the IMF (Fig. 3C). Subsequently, a new population of highly relativistic electrons emerged at a region around L* ~4.0 and grew in intensity and spatial extent (Fig. 3A) after a high-speed solar wind episode (Fig. 3B) on 5 September. Another such period of high-energy electron flux diminution, reappearance, and intensification was seen from ~21 September to 1 October 2012 (Fig. 3A), with this sequence again occurring in the wake of a powerful high-speed solar wind stream on 20 to 21 September (Fig. 3B). As noted above, one of the most abrupt and notable features of the entire data set was the nearly complete disappearance of the entire outer-zone electron population late on 1 October associated with another interplanetary shock wave (Fig. 3, B and C) and relatively strong geomagnetic storm [seen in disturbance storm time (Dst), which measures global magnetic field disturbance (Fig. 3D)].

Fig. 3 Development of the storage ring.

(A) Image similar to Fig. 1B, but also including the plasmapause, the outer boundary of the plasmasphere (26) for the period 1 September to 7 October 2012. The white curve overplotted on the color-coded electron particle flux data in Fig. 3A shows the modeled, 3-day averaged plasmapause radial location that is in agreement with concurrent plasma wave data (17, 27, 28). (B) Concurrently measured solar wind speed upstream of Earth’s magnetosphere. (C) Interplanetary magnetic field for the interval under study. |B|, the magnitude of the interplanetary magnetic field components parallel to the ecliptic in nanotesla (nT); Bz, the values of the components perpendicular. (D) Geomagnetic activity index (Dst) for the period under study.

Figure 3A shows that for the period of 1 to 4 September 2012, the average plasmapause boundary was relatively close to Earth (L* ~ 3), and a powerful outer-zone electron acceleration event was occurring in the low–plasma-density region outside the plasmasphere. However, from ~4 September until ~6 October, the plasmapause was much farther outward, around L* > 4. Thus, the storage-ring feature, as well as most of the outer Van Allen zone E > 4.5 MeV electron population, was inside the high-density plasmasphere. However, in the traditional picture, the outer-zone electron belt would be largely outside the plasmasphere, and the slot region would be inside the plasmasphere outer boundary (2123, 25).

The radiation belt particle populations are determined by a complex superposition of acceleration, transport, and loss processes modulated by their interactions with plasma waves (24). We are now seeing unexpected radiation belt structures (Fig. 4), but have yet to fully understand them in the context of present radiation belt theory.

Fig. 4 Radiation belt structures.

Diagrams providing a cross-sectional view of Earth’s radiation belt structure and relation to the plasmasphere. (A) Schematic diagram showing Earth, the outer and inner radiation belts, and the normal plasmaspheric location. (B) Similar to (A), but showing a more highly distended plasmasphere and unexpected triple radiation belt properties during the September 2012 period. The radiation belts are really “doughnut-” or torus-shaped entities in three dimensions. Earth is portrayed at the center. White denotes the highest electron fluxes; blue indicates the lowest fluxes. The translucent green overlay denotes the plasmasphere.

Supplementary Materials

www.sciencemag.org/cgi/content/full/science.1233518/DC1

Supplementary Text

Figs. S1 and S2

References

References and Notes

  1. Since the initial Van Allen belt discovery, there have been many missions that have measured key aspects of the radiation properties around Earth. Some of these have been from operational satellite systems such as the National Oceanic and Atmospheric Administration weather satellites in geostationary Earth orbit (GEO) (www.oso.noaa.gov/goesstatus) or polar low-Earth (www.oso.noaa.gov/poesstatus) orbits. Other measurements have been made using sensors on board operational GEO spacecraft or the Global Positioning Satellite timing and navigation constellation of spacecraft, as well as the Polar and Cluster scientific satellites (12). These prior satellites have provided key long-term monitoring of radiation belt changes, but have generally not made measurements directly in the heart of the radiation belt regions. Only the CRRES mission (13) operated briefly (1990 to 1991) in the heart of the radiation belts, but this mission lacked the background rejection and the temporal, energy, and spatial resolution now provided by the dual Van Allen Probes.
  2. See data and methods in the accompanying supplementary materials on Science Online.
  3. Acknowledgments: This work was supported by RBSP–Energetic Particle Composition and Thermal Plasma Suite funding provided by the Johns Hopkins University Applied Physics Laboratory (JHU/APL) contract no. 967399, Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) work was supported on JHU/APL contract no. 921649, and both were funded under NASA’s Prime contract no. NAS5-01072. All Van Allen Probes observations used in this study, along with display and analysis software, are publicly available at the Web site www.rbsp-ect.lanl.gov.
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